Laminated body

ABSTRACT

To afford a laminated body that is usable as a nonaqueous electrolyte secondary battery separator and that is not easily curled, a laminated body includes: a porous base material containing a polyolefin-based resin as a main component; and a porous layer containing a polyvinylidene fluoride-based resin, the porous base material having a temperature rise ending period of a particular value with respect to the amount of resin per unit area, the polyvinylidene fluoride-based resin containing crystal form α in an amount of not less than 36 mol % with respect to 100 mol % of a total amount of the crystal form α and crystal form β contained in the polyvinylidene fluoride-based resin.

This Nonprovisional application claims priority under 35 U.S.C. § 119 onPatent Application No. 2016-123055 filed in Japan on Jun. 21, 2016, theentire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a laminated body, and morespecifically, to a laminated body usable as a separator for a nonaqueouselectrolyte secondary battery (hereinafter referred to as a “nonaqueouselectrolyte secondary battery separator”).

BACKGROUND ART

Nonaqueous electrolyte secondary batteries such as a lithium-ionsecondary battery have a high energy density, and are thus in wide useas batteries for devices such as a personal computer, a mobiletelephone, and a portable information terminal. Such nonaqueouselectrolyte secondary batteries have recently been developed ason-vehicle batteries.

In a nonaqueous electrolyte secondary battery, the electrodes expand andcontract repeatedly as the nonaqueous electrolyte secondary battery ischarged and discharged. The electrodes and the separator thus causestress on each other. This, for example, causes the electrode activematerials to fall off and consequently increases the internalresistance, unfortunately resulting in a degraded cycle characteristic.In view of that, there has been proposed a technique of coating thesurface of a separator with an adhesive material such as polyvinylidenefluoride for increased adhesiveness between the separator and electrodes(see Patent Literatures 1 and 2). Coating a separator with an adhesivematerial, however, has been causing the separator to curl visibly. Acurled separator cannot be handled easily during production, which mayunfortunately lead to problems during battery preparation such asdefective rolling and defective assembly.

CITATION LIST Patent Literature

[Patent Literature 1] Japanese Patent No. 5355823 (Publication Date:Nov. 27, 2013)

[Patent Literature 2] Japanese Patent Application Publication, Tokukai,No. 2001-118558 (Publication Date: Apr. 27, 2001)

SUMMARY OF INVENTION Technical Problem

The present invention has been accomplished in view of the above issue.It is an object of the present invention to sufficiently prevent aseparator from curling.

Solution to Problem

In order to attain the above object, the inventor of the presentinvention has conducted diligent research and have thus discovered thata separator capable of sufficiently preventing itself from curling canbe produced from a laminated body including (i) a porous base materialcontaining a polyolefin resin as a main component and (ii) a porouslayer disposed on the porous base material which porous layer contains apolyvinylidene fluoride-based resin (hereinafter also referred to as“PVDF-based resin”), the polyvinylidene fluoride-based resin havingmoderately controlled crystal forms. Further, the porous base materialhas a temperature rise ending period of 2.9 seconds·m²/g to 5.7seconds·m²/g with respect to an amount of resin per unit area in a casewhere the porous base material has been impregnated withN-methylpyrrolidone containing 3% by weight of water and has then beenirradiated with a microwave having a frequency of 2455 Hz and an outputof 1800 W.

The laminated body in accordance with an embodiment of the presentinvention may preferably be arranged such that the polyvinylidenefluoride-based resin includes (i) a homopolymer of vinylidene fluorideand/or (ii) a copolymer of vinylidene fluoride and at least one monomerselected from the group consisting of hexafluoropropylene,tetrafluoroethylene, trifluoroethylene, trichloroethylene, and vinylfluoride.

The laminated body in accordance with an embodiment of the presentinvention may preferably be arranged such that the polyvinylidenefluoride-based resin has a weight-average molecular weight of not lessthan 200,000 and not more than 3,000,000.

The laminated body in accordance with an embodiment of the presentinvention may preferably be arranged such that the porous layer containsa filler.

The laminated body in accordance with an embodiment of the presentinvention may preferably be arranged such that the filler has avolume-average particle size of not less than 0.01 μm and not more than10 μm.

A member for a nonaqueous electrolyte secondary battery (hereinafterreferred to as a “nonaqueous electrolyte secondary battery member”) inaccordance with an embodiment of the present invention includes: acathode; a laminated body in accordance with an embodiment of thepresent invention; and an anode, the cathode, the laminated body, andthe anode being arranged in this order.

A nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention includes as a separator a laminatedbody in accordance with an embodiment of the present invention.

Advantageous Effects of Invention

An embodiment of the present invention can advantageously prevent curls.

DESCRIPTION OF EMBODIMENTS

The following description will discuss an embodiment of the presentinvention. The present invention is, however, not limited to theembodiment below. The present invention is not limited to thearrangements described below, but may be altered in various ways by askilled person within the scope of the claims. Any embodiment based on aproper combination of technical means disclosed in different embodimentsis also encompassed in the technical scope of the present invention.Note that numerical expressions such as “A to B” herein mean “not lessthan A and not more than B” unless otherwise stated.

[1. Laminated Body]

A laminated body of the present embodiment includes: a porous basematerial containing a polyolefin-based resin as a main component; and aporous layer on at least one surface of the porous base material, theporous layer containing a polyvinylidene fluoride-based resin, theporous base material having a temperature rise ending period of 2.9seconds·m²/g to 5.7 seconds·m²/g with respect to an amount of resin perunit area in a case where the porous base material has been impregnatedwith N-methylpyrrolidone containing 3% by weight of water and has thenbeen irradiated with a microwave having a frequency of 2455 Hz and anoutput of 1800 W, the polyvinylidene fluoride-based resin containingcrystal form α in an amount of not less than 36 mol % with respect to100 mol % of the total amount of the crystal form α and crystal form βcontained in the polyvinylidene fluoride-based resin, where the amountof the crystal form α is calculated from an absorption intensity ataround 765 cm⁻¹ in an IR spectrum of the porous layer, and an amount ofthe crystal form β is calculated from an absorption intensity at around840 cm⁻¹ in the IR spectrum of the porous layer.

The following description will discuss individual components.

[1-1. Porous Base Material]

The porous base material in accordance with an embodiment of the presentinvention is a porous film included in a nonaqueous electrolytesecondary battery and disposed between the cathode and the anode.

The porous base material simply needs to be a porous, filmy basematerial containing a polyolefin-based resin as a main component(polyolefin-based porous base material). The porous base material is afilm that contains pores connected to one another and that allows a gas,a liquid, or the like to pass therethrough from one surface to theother.

The term “main component” intends to mean accounting for not less than50% by volume of the porous base material in its entirety.

The porous base material is arranged such that in a case where thebattery generates heat, the porous film is melted so as to make thelaminated body (which functions as a nonaqueous electrolyte secondarybattery separator) non-porous. This allows the porous base material toimpart a shutdown function to the laminated body (which functions as anonaqueous electrolyte secondary battery separator). The porous basematerial may include a single layer or a plurality of layers.

The inventors of the present invention have discovered a relationshipbetween (i) a time period (temperature rise ending period) that elapsesbefore the end of a rise in the temperature of a porous base material ina case where the porous base material has been impregnated withN-methylpyrrolidone containing 3% by weight of water and has then beenirradiated with a microwave having a frequency of 2455 Hz and an outputof 1800 W and (ii) the initial rate characteristic of a batteryincluding the porous base material and how the rate characteristic isdegraded as the battery is charged and discharged repeatedly.

When a nonaqueous electrolyte secondary battery is charged anddischarged, the electrodes expand. Specifically, the anode expands whenthe battery is charged, whereas the cathode expands when the battery isdischarged. Thus, the electrolyte inside the porous base material ispushed out from the side of the expanding electrode to the side of theopposite electrode. This mechanism causes the electrolyte to move outfrom and into the porous base material during a charge and dischargecycle. Since the porous base material contains pores as described above,the electrolyte moves in and out through those pores.

When the electrolyte moves through the pores of the porous basematerial, the wall surface of the pores is subjected to stress caused bythe movement. The strength of the stress is related to the structure ofthe pores, that is, the capillary force in the pores connected to oneanother and the area of the wall of the pores. Specifically, the wallsurface of the pores is subjected to stress that is presumably higherwith a stronger capillary force and with a larger area of the wallsurface of the pores. In addition, the strength of the stress is alsorelated to the amount of electrolyte moving through the pores: thestress is presumably stronger with a larger amount of electrolyte movingthrough the pores, that is, in a case where the battery is used with alarger electric current. An increase in the stress deforms the wallsurface in such a manner that the wall surface blocks the pores, withthe result of degradation in the battery output characteristic. The ratecharacteristic is thus gradually degraded as the battery is charged anddischarged repeatedly and/or the battery is used with a large electriccurrent.

In a case where the electrolyte is pushed out from the porous basematerial in only a small amount, the amount of electrolyte with respectto an electrode surface is decreased, or the electrolyte is dried uplocally on an electrode surface. This may lead to an increase in theamount of a degradation product of the electrolyte. Such a degradationproduct of the electrolyte can cause a nonaqueous electrolyte secondarybattery to have a degraded rate characteristic.

As described above, there is a relationship between (i) the structure ofpores of a porous base material (namely, the capillary force in thepores and the area of the wall of the pores) and the capability tosupply an electrolyte from the porous base material to the electrodesand (ii) how the rate characteristic is degraded as the battery ischarged and discharged repeatedly and/or the battery is used with alarge electric current. In view of that, the inventors of the presentinvention focused on how the temperature of a porous base materialchanges in a case where the porous base material has been impregnatedwith N-methylpyrrolidone containing 3% by weight of water and has thenbeen irradiated with a microwave having a frequency of 2455 Hz and anoutput of 1800 W.

Irradiating a porous base material containing N-methylpyrrolidonecontaining water with a microwave causes the porous base material togenerate heat due to vibrational energy of the water. The heat generatedis transmitted to the resin contained in the porous base material whichresin is in contact with the N-methylpyrrolidone containing water. Thetemperature rise ends when equilibrium is reached between (i) the rateof heat generation and (ii) the rate of cooling due to transfer of heatto the resin. This indicates that the time period that elapses beforethe end of a temperature rise (that is, the temperature rise endingperiod) is related to the degree of contact between (i) the liquidcontained in the porous base material (in this example,N-methylpyrrolidone containing water) and (ii) the resin contained inthe porous base material. The degree of contact between the liquidcontained in the porous base material and the resin contained in theporous base material is closely related to the capillary force in thepores of the porous base material and the area of the wall of the pores.Thus, the temperature rise ending period can be used for evaluation ofthe structure of pores of a porous base material (namely, the capillaryforce in the pores and the area of the wall of the pores). Specifically,a shorter temperature rise ending period indicates that the capillaryforce in the pores is larger and that the area of the wall of the poresis larger.

The degree of contact between the liquid contained in the porous basematerial and the resin contained in the porous base material ispresumably larger in a case where liquid moves more easily through thepores of the porous base material. This makes it possible to use thetemperature rise ending period for evaluation of the capability tosupply an electrolyte from the porous base material to the electrodes.Specifically, a shorter temperature rise ending period indicates ahigher capability to supply an electrolyte from the porous base materialto the electrodes.

A porous base material for an embodiment of the present invention has atemperature rise ending period of 2.9 seconds·m²/g to 5.7 seconds·m²/g,preferably 2.9 seconds·m²/g to 5.3 seconds·m²/g, with respect to theamount of resin per unit area (weight per unit area).

If the temperature rise ending period with respect to the amount ofresin per unit area is less than 2.9 seconds·m²/g, both the capillaryforce in the pores of the porous base material and the area of the wallof the pores will be excessively large. This will lead to an increase inthe stress caused on the wall of the pores when the electrolyte movesthrough the pores during a charge and discharge cycle and/or during useof the battery with a large electric current. This will in turn blockthe pores, with the result of degradation in the battery outputcharacteristic.

If the temperature rise ending period with respect to the amount ofresin per unit area is more than 5.7 seconds·m²/g, liquid will move lesseasily through the pores of the porous base material, and in a casewhere the porous base material is used as a separator for a nonaqueouselectrolyte secondary battery, the electrolyte will move more slowlynear the interface between the porous base material and an electrode,with the result of a decrease in the rate characteristic of the battery.In addition, when the battery has been charged and dischargedrepeatedly, the electrolyte will be more likely dried up locally at theinterface between the separator and an electrode or inside the porousbase material. This will in turn lead to an increase in the internalresistance of the battery, with the result of a nonaqueous electrolytesecondary battery having a rate characteristic that is degraded througha charge and discharge cycle.

In contrast, a temperature rise ending period of 2.9 seconds·m²/g to 5.7seconds·m²/g with respect to the amount of resin per unit area allowsfor an excellent initial rate characteristic and prevents the ratecharacteristic from being degraded through a charge and discharge cycle,as demonstrated in the Examples described later.

The porous base material may have any film thickness that is selected asappropriate in view of the thickness of a nonaqueous electrolytesecondary battery member to be included in a nonaqueous electrolytesecondary battery to be produced. The porous base material has a filmthickness of preferably 4 μm to 40 μm, more preferably 5 μm to 30 μm,further preferably 6 μm to 15 μm

The porous base material has a volume-based porosity of 20% by volume to80% by volume, preferably 30% by volume to 75% by volume, so as to (i)retain a larger amount of electrolyte and (ii) obtain the function ofreliably preventing (shutting down) a flow of an excessively largeelectric current at a lower temperature. Further, in order to obtainsufficient ion permeability and prevent particles from entering thecathode and/or the anode in a case where the laminated body is used as aseparator, the porous base material has pores having an average diameter(average pore diameter) of preferably not more than 0.3 μm, morepreferably not more than 0.14 μm.

The porous base material contains a polyolefin component at a proportionof not less than 50% by volume, preferably not less than 90% by volume,more preferably not less than 95% by volume, with respect to the entireporous base material. The porous base material preferably contains, asthe polyolefin component, a high molecular weight component having aweight-average molecular weight of 5×10⁵ to 15×10⁶. The porous basematerial preferably contains a polyolefin component having aweight-average molecular weight of not less than 1,000,000 because sucha polyolefin component allows the porous base material and a nonaqueouselectrolyte secondary battery separator in its entirety to have a higherstrength.

Examples of the polyolefin-based resin contained in the porous basematerial include high molecular weight homopolymers and copolymersproduced through polymerization of ethylene, propylene, 1-butene,4-methyl-1-pentene, and 1-hexene. The porous base material can include alayer containing only one of these polyolefin-based resins and/or alayer containing two or more of these polyolefin-based resins. Thepolyolefin-based resin is, in particular, preferably a high molecularweight polyethylene containing ethylene as a main component. The porousbase material may contain a component(s) other than polyolefin as longas such a component does not impair the function of the layer.

Examples of the polyethylene-based resin include low-densitypolyethylene, high-density polyethylene, linear polyethylene(ethylene-a-olefin copolymer), and ultra-high molecular weightpolyethylene having a weight-average molecular weight of not less than1,000,000. Among these examples, ultra-high molecular weightpolyethylene having a weight-average molecular weight of not less than1,000,000 is further preferable.

The porous base material has an air permeability within a range ofnormally 30 seconds/100 cc to 500 seconds/100 cc, preferably 50seconds/100 cc to 300 seconds/100 cc, in terms of Gurley values. Aporous base material having an air permeability within the above rangeallows for sufficient ion permeability in a case where the laminatedbody is used as a separator.

The porous base material has a weight per unit area of preferably 4 g/m²to 20 g/m², more preferably 4 g/m² to 12 g/m², even more preferably 5g/m² to 12 g/m². This is because such a weight per unit area of theporous base material can increase (i) the strength, thickness, handlingeasiness, and weight of the porous base material and (ii) the weightenergy density and volume energy density of a nonaqueous electrolytesecondary battery including the laminated body as a nonaqueouselectrolyte secondary battery separator.

The physical property values of the porous base material, which isincluded in a laminated body including the porous base material and aporous layer, can be measured after the porous layer is removed from thelaminated body. The porous layer can be removed from the laminated bodyby, for example, a method of dissolving the resin of the porous layerwith use of a solvent such as N-methylpyrrolidone or acetone forremoval.

The following description will discuss a method for producing a porousbase material. The porous base material containing a polyolefin-basedresin as a main component, for example, a porous base materialcontaining (i) ultra-high molecular weight polyethylene and (ii) apolyolefin resin containing a low molecular weight polyolefin having aweight-average molecular weight of not more than 10,000, is preferablyproduced by a method as described below.

Specifically, the porous base material can be obtained by a methodincluding the steps of (1) obtaining a polyolefin resin composition bykneading (i) ultra-high molecular weight polyethylene, (ii) a lowmolecular weight polyolefin having a weight-average molecular weight ofnot more than 10,000, and (iii) a pore forming agent such as calciumcarbonate or a plasticizing agent, (2) forming (rolling) a sheet withuse of a reduction roller to roll the polyolefin resin compositionobtained in the step (1), (3) removing the pore forming agent from thesheet obtained in the step (2), and (4) obtaining a porous base materialby stretching the sheet obtained in the step (3).

The structure of pores of a porous base material (namely, the capillaryforce of the pores, the area of the wall of the pores, and stressremaining in the porous base material) is influenced by the strainingrate during the stretching in the step (4) and the temperature during aheat-fixation treatment (annealing treatment) after the stretching perunit thickness of the stretched film (that is, a heat-fixationtemperature per unit thickness of the stretched film). Thus, in a casewhere the straining rate and the heat-fixation temperature per unitthickness of the stretched film have been adjusted, controlling thestructure of the pores of the porous base material makes it possible tocontrol the temperature rise ending period with respect to the amount ofresin per unit area.

Specifically, a porous base material for an embodiment of the presentinvention tends to be produced in a case where the straining rate andthe heat-fixation temperature per unit thickness of the stretched filmhave been so adjusted as to be, on a graph having an X axis indicativeof the straining rate and a Y axis indicative of the heat-fixationtemperature per unit thickness of the stretched film, within atriangular area having three vertices at (i) 500% per minute and 1.5°C./μm (ii) 900% per minute and 14.0° C./μm, and (iii) 2500% per minuteand 11.0° C./μm. The straining rate and the heat-fixation temperatureper unit thickness of the stretched film are preferably so adjusted asto be within a triangular area having three vertices at (i) 600% perminute and 5.0° C./μm (ii) 900% per minute and 12.5° C./μm, and (iii)2500% per minute and 11.0° C./μm.

[1-2. Porous Layer]

The porous layer is disposed on one surface or both surfaces of theporous base material as a porous film as necessary. It is preferablethat the resin of which the porous layer is made be insoluble in theelectrolyte of the battery and be electrochemically stable when thebattery is in normal use. In a case where the porous layer is disposedon one surface of the porous base material, the porous layer is disposedpreferably on a surface of the porous base material which surface facesthe cathode of the nonaqueous electrolyte secondary battery, morepreferably on a surface of the porous base material which surface is incontact with the cathode.

The porous layer for an embodiment of the present invention contains apolyvinylidene fluoride-based resin, the polyvinylidene fluoride-basedresin containing crystal form α in an amount of not less than 36 mol %with respect to 100 mol % of the total amount of the crystal form α andcrystal form β contained in the polyvinylidene fluoride-based resin.

The amount of crystal form α is calculated from the absorption intensityat around 765 cm⁻¹ in the IR spectrum of the porous layer, while theamount of crystal form β is calculated from the absorption intensity ataround 840 cm⁻¹ in the IR spectrum of the porous layer.

The porous layer for an embodiment of the present invention contains apolyvinylidene fluoride-based resin (PVDF-based resin). The porous layercontains a large number of pores connected to one another, and thusallows a gas or a liquid to pass therethrough from one surface to theother. Further, in a case where the porous layer for an embodiment ofthe present invention is used as a constituent member of a nonaqueouselectrolyte secondary battery separator, the porous layer can be a layercapable of adhering to an electrode as the outermost layer of theseparator.

Examples of the PVDF-based resin include homopolymers of vinylidenefluoride (that is, polyvinylidene fluoride); copolymers (for example,polyvinylidene fluoride copolymer) of vinylidene fluoride and othermonomer(s) polymerizable with vinylidene fluoride; and mixtures of theabove polymers. Examples of the monomer polymerizable with vinylidenefluoride include hexafluoropropylene, tetrafluoroethylene,trifluoroethylene, trichloroethylene, and vinyl fluoride. An embodimentof the present invention can use (i) one kind of monomer or (ii) two ormore kinds of monomers selected from above. The PVDF-based resin can besynthesized through emulsion polymerization or suspensionpolymerization.

The PVDF-based resin contains vinylidene fluoride at a proportion ofnormally not less than 85 mol %, preferably not less than 90 mol %, morepreferably not less than 95 mol %, further preferably not less than 98mol %. A PVDF-based resin containing vinylidene fluoride at a proportionof not less than 85 mol % is more likely to allow a porous layer to havea mechanical strength against pressure and a heat resistance againstheat during battery production.

The porous layer can also preferably contain two kinds of PVDF-basedresins (that is, a first resin and a second resin below) that differfrom each other in terms of, for example, the hexafluoropropylenecontent.

The first resin is (i) a vinylidene fluoride-hexafluoropropylenecopolymer containing hexafluoropropylene at a proportion of more than 0mol % and not more than 1.5 mol % or (ii) a vinylidene fluoridehomopolymer (containing hexafluoropropylene at a proportion of 0 mol %).

The second resin is a vinylidene fluoride-hexafluoropropylene copolymercontaining hexafluoropropylene at a proportion of more than 1.5 mol %.

A porous layer containing the two kinds of PVDF-based resins adheresbetter to an electrode than a porous layer not containing one of the twokinds of PVDF-based resins. Further, a porous layer containing the twokinds of PVDF-based resins adheres better to another layer (for example,the porous base material layer) included in a nonaqueous electrolytesecondary battery separator, with the result of a higher peel forcebetween the two layers, than a porous layer not containing one of thetwo kinds of PVDF-based resins. The first resin and the second resin arepreferably mixed at a mixing ratio (mass ratio, first resin:secondresin) of 15:85 to 85:15.

The PVDF-based resin has a weight-average molecular weight of preferably200,000 to 3,000,000. A PVDF-based resin having a weight-averagemolecular weight of not less than 200,000 tends to allow a porous layerto attain a mechanical property enough for the porous layer to endure aprocess of adhering the porous layer to an electrode, thereby allowingthe porous layer and the electrode to adhere to each other sufficiently.A PVDF-based resin having a weight-average molecular weight of not morethan 3,000,000 tends to not cause the coating solution, which is to beapplied to form a porous layer, to have too high a viscosity, whichallows the coating solution to have excellent shaping easiness. Theweight-average molecular weight of the PVDF-based resin is morepreferably 200,000 to 2,000,000, further preferably 500,000 to1,500,000.

The PVDF-based resin has a fibril diameter of preferably 10 nm to 1000nm in view of the cycle characteristic of a nonaqueous electrolytesecondary battery containing the porous layer.

The porous layer for an embodiment of the present invention may containa resin other than the PVDF-based resin. Examples of the other resininclude styrene-butadiene copolymers; homopolymers or copolymers ofvinyl nitriles such as acrylonitrile and methacrylonitrile; andpolyethers such as polyethylene oxide and polypropylene oxide.

The porous layer for an embodiment of the present invention may containa filler. The filler may be an inorganic or organic filler. In a casewhere the porous layer for an embodiment of the present inventioncontains a filler, the filler is contained at a proportion of preferablynot less than 1% by mass and not more than 99% by mass, more preferablynot less than 10% by mass and not more than 98% by mass, with respect tothe total amount of the polyvinylidene fluoride-based resin and thefiller combined. Containing a filler allows a separator including theporous layer to have improved slidability and heat resistance, forexample. The filler may be any inorganic or organic filler that isstable in a nonaqueous electrolyte and that is stable electrochemically.The filler preferably has a heat-resistant temperature of not lower than150° C. to ensure safety of the battery.

Examples of the organic filler include: crosslinked polyacrylic acid,crosslinked polyacrylic acid ester, crosslinked polymethacrylic acid,and crosslinked polymethacrylic acid esters such as crosslinkedpolymethyl methacrylate; fine particles of crosslinked polymers such ascrosslinked polysilicone, crosslinked polystyrene, crosslinkedpolydivinyl benzene, a crosslinked product of a styrene-divinylbenzenecopolymer, polyimide, a melamine resin, a phenol resin, and abenzoguanamine-formaldehyde condensate; and fine particles ofheat-resistant polymers such as polysulfone, polyacrylonitrile,polyaramid, polyacetal, and thermoplastic polyimide.

A resin (polymer) contained in the organic filler may be a mixture, amodified product, a derivative, a copolymer (a random copolymer, analternating copolymer, a block copolymer, or a graft copolymer), or acrosslinked product of any of the molecular species listed above asexamples.

Specific examples of the organic filler also include fillers made of (i)a homopolymer of a monomer such as styrene, vinyl ketone, acrylonitrile,methyl methacrylate, ethyl methacrylate, glycidyl methacrylate, glycidylacrylate, or methyl acrylate, or (ii) a copolymer of two or more of suchmonomers; fluorine-containing resins such as polytetrafluoroethylene, anethylene tetrafluoride-propylene hexafluoride copolymer, an ethylenetetrafluoride-ethylene copolymer, and polyvinylidene fluoride; melamineresin; urea resin; polyethylene; polypropylene; polyacrylic acid andpolymethacrylic acid; and the like.

Examples of the inorganic filler include metal hydroxides such asaluminum hydroxide, magnesium hydroxide, calcium hydroxide, chromiumhydroxide, zirconium hydroxide, nickel hydroxide, and boron hydroxide;metal oxides such as alumina and zirconia, and hydrates thereof;carbonates such as calcium carbonate and magnesium carbonate; sulfatessuch as barium sulfate and calcium sulfate; and clay minerals such ascalcium silicate and talc. The inorganic filler is preferably a metalhydroxide, a hydrate of a metal oxide, or a carbonate to improve thesafety of the battery, for example, to impart fire retardance. Theinorganic filler is preferably a metal oxide in terms of insulation andoxidation resistance.

Specific examples of the inorganic filler also include fillers made ofinorganic matters such as calcium carbonate, talc, clay, kaolin, silica,hydrotalcite, diatomaceous earth, magnesium carbonate, barium carbonate,calcium sulfate, magnesium sulfate, barium sulfate, aluminum hydroxide,magnesium hydroxide, calcium oxide, magnesium oxide, titanium oxide,titanium nitride, alumina (aluminum oxide), aluminum nitride, mica,zeolite, and glass.

An embodiment of the present invention may use (i) only one filler or(ii) two or more kinds of fillers in combination. Alternatively, theorganic filler(s) and the inorganic filler(s) may be used incombination.

The filler has a volume average particle size of preferably 0.01 μm to10 μm in order to ensure (i) fine adhesion and fine slidability and (ii)shaping easiness of the laminated body. The volume average particle sizehas a lower limit of more preferably not less than 0.05 μm, furtherpreferably not less than 0.1 μm. The volume average particle size has anupper limit of more preferably not more than 5 μm, further preferablynot more than 1 μm.

The filler may have any shape. The filler may, for example, be aparticulate filler. Example shapes of the particles include a sphere, anellipse, a plate shape, a bar shape, and an irregular shape. In order toprevent a short circuit in the battery, the filler is preferably in theform of (i) plate-shaped particles or (ii) primary particles that arenot aggregated.

The filler forms fine bumps on a surface of the porous layer, therebyimproving the slidability. A filler including (i) plate-shaped particlesor (ii) primary particles that are not aggregated forms finer bumps on asurface of the porous layer so that the porous layer adheres better toan electrode.

The porous layer for an embodiment of the present invention has anaverage thickness of preferably 0.5 μm to 10 μm, more preferably 1 μm to5 μm, on one surface of the porous base material in order to ensureadhesion to an electrode and a high energy density.

<Crystal Forms of PVDF-Based Resin>

The PVDF-based resin in the porous layer for an embodiment of thepresent invention contains crystal form α in an amount of not less than36 mol %, preferably not less than 39 mol %, more preferably not lessthan 40 mol %, more preferably not less than 50 mol %, more preferablynot less than 60 mol %, further preferably not less than 70 mol %, withrespect to 100 mol % of the total amount of crystal form α and crystalform β contained. Further, the amount of crystal form α is preferablynot more than 95 mol %. Containing crystal form α in an amount of notless than 36 mol % allows a laminated body including the porous layer tobe used as a member of a nonaqueous electrolyte secondary battery suchas a nonaqueous electrolyte secondary battery separator that is noteasily curled.

A laminated body in accordance with an embodiment of the presentinvention can prevent itself from curling presumably because, forexample, (i) a smaller content of the PVDF-based resin having crystalform β, which PVDF-based resin strongly adheres to the porous basematerial, allows the porous layer to be deformed to only a moderatelysmaller degree in response to deformation of the porous base materialand/or (ii) a larger content of the PVDF-based resin having crystal formα, which PVDF-based resin is high in rigidity, allows the porous layerto be more resistant to deformation.

The PVDF-based resin having crystal form α is arranged such that in thePVDF skeleton contained in the polymer of the PVDF-based resin, withrespect to a fluorine atom (or a hydrogen atom) bonded to a singlemain-chain carbon atom in the molecular chains contained in the PVDFskeleton, one adjacent carbon atom is bonded to a hydrogen atom (or afluorine atom) having a trans position relative to the above fluorineatom (or the above hydrogen atom), and the other (opposite) adjacentcarbon atom is bonded to a hydrogen atom (or a fluorine atom) having agauche position (positioned at an angle of 60°) relative to the abovefluorine atom (or the above hydrogen atom), wherein two or more suchconformations are chained consecutively as follows:(TGTG structure)   [Math. 1]

and the molecular chains each have the following type:TGTG  [Math. 2]

wherein the respective dipole moments of C—F₂ and C—H₂ bonds each have acomponent perpendicular to the molecular chain and a component parallelto the molecular chain.

The PVDF-based resin having crystal form α has characteristic peaks(characteristic absorptions) at around 1,212 cm⁻¹, around 1,183 cm⁻¹,and around 765 cm⁻¹ in its IR spectrum. The PVDF-based resin havingcrystal form α has characteristic peaks at around 2θ=17.7°, around2θ=18.3°, and around 2θ=19.9° in a powder X-ray diffraction analysis.

The PVDF-based resin having crystal form β is arranged such that in thePVDF skeleton contained in the polymer of the PVDF-based resin, amain-chain carbon atom in the molecular chains contained in the PVDFskeleton is adjacent to two carbon atoms bonded to a fluorine atom and ahydrogen atom, respectively, having a trans conformation (TT-typeconformation), that is, the fluorine atom and the hydrogen atom bondedrespectively to the two adjacent carbon atoms are positioned at an angleof 180° to the direction of the carbon-carbon bond.

The PVDF-based resin having crystal form β may be arranged such that thepolymer of the PVDF-based resin contains a PVDF skeleton that has aTT-type conformation in its entirety. The PVDF-based resin havingcrystal form β may alternatively be arranged such that a portion of thePVDF skeleton has a TT-type conformation and that the PVDF-based resinhaving crystal form β has a molecular chain of the TT-type conformationin at least four consecutive PVDF monomeric units. In either case, (i)the carbon-carbon bond, in which the TT-type conformation constitutes aTT-type main chain, has a planar zigzag structure, and (ii) therespective dipole moments of C—F₂ and C—H₂ bonds each have a componentperpendicular to the molecular chain.

The PVDF-based resin having crystal form β has characteristic peaks(characteristic absorptions) at around 1,274 cm⁻¹, around 1,163 cm⁻¹,and around 840 cm⁻¹ in its IR spectrum. The PVDF-based resin havingcrystal form β has a characteristic peak at around 2θ=21° in a powderX-ray diffraction analysis.

A PVDF-based resin having crystal form γ is arranged such that thepolymer of the PVDF-based resin contains a PVDF skeleton that has aconformation in which TT-type conformations and TG-type conformationsappear consecutively and alternately. The PVDF-based resin havingcrystal form γ has characteristic peaks (characteristic absorptions) ataround 1,235 cm⁻¹ and around 811 cm⁻¹ in its IR spectrum. The PVDF-basedresin having crystal form γ has a characteristic peak at around 2θ=18°in a powder X-ray diffraction analysis.

<Method for Calculating Content Rates of Crystal Form α and Crystal Formβ in PVDF-Based Resin>

The respective content rates of crystal form α and crystal form β in thePVDF-based resin can be calculated by, for example, the methods (i) to(iii) below.

(i) Calculation FormulaLaw of Beer: A=εbC   (1)

where A represents an absorbance, ε represents a molar extinctioncoefficient, b represents an optical path length, and C represents aconcentration.

Assuming that on the basis of the above formula (1), A^(α) representsthe absorbance of the characteristic absorption of crystal form α, A^(β)represents the absorbance of the characteristic absorption of crystalform β, ε^(α) represents the molar extinction coefficient of thePVDF-based resin having crystal form α, ε^(β) represents the molarextinction coefficient of the PVDF-based resin having crystal form β,C^(α) represents the concentration of the PVDF-based resin havingcrystal form α, and C^(β) represents the concentration of the PVDF-basedresin having crystal form β, the respective proportions of therespective absorbances of crystal form α and crystal form β areexpressed as follows:A ^(β) /A ^(α)=(ε^(β)/ε^(α))×(C ^(β) /C ^(α))   (1a)

Assuming that a correction factor (ε^(β)/ε^(α)) for the molar extinctioncoefficient is E^(β/α), the content rate F(β)=(C^(β)/(C^(α)+C^(β))) ofthe PVDF-based resin having crystal form β with respect to the crystalform α and crystal form β combined is expressed by the following formula(2a):

$\begin{matrix}{{F(\beta)} = {{\left\{ {\left( {1/E^{\beta/\alpha}} \right) \times \left( {A^{\alpha}/A^{\beta}} \right)} \right\}/\left\{ {1 + {\left( {1/E^{\beta/\alpha}} \right) \times \left( {A^{\alpha}/A^{\beta}} \right)}} \right\}} = {A^{\beta}/\left\{ {\left( {E^{\beta/\alpha} \times A^{\alpha}} \right) + A^{\beta}} \right\}}}} & \left( {2a} \right)\end{matrix}$

Thus, in a case where the correction factor E^(β/α) is determined, thecontent rate F(β) of the PVDF-based resin having crystal form β withrespect to the crystal form α and crystal form β combined can becalculated from an actual measurement of the absorbance A^(α) of thecharacteristic absorption of crystal form α and an actual measurement ofthe absorbance A^(β) of the characteristic absorption of crystal form β.Further, the content rate F(α) of the PVDF-based resin having crystalform α with respect to the crystal form α and crystal form β combinedcan be calculated from F(β) calculated as above.

(ii) Method for Determining Correction Factor E^(β/α)

A sample of a PVDF-based resin having only crystal form α is mixed witha sample of a PVDF-based resin having only crystal form β forpreparation of a sample with a known content rate F(β) of the PVDF-basedresin having crystal form β. The IR spectrum of the prepared sample ismeasured. Then, measurements are made of the absorbance (peak height)A^(α) of the characteristic absorption of crystal form α and theabsorbance (peak height) A^(β) of the characteristic absorption ofcrystal form β in the IR spectrum measured above.

Subsequently, A^(α) and A^(β) are substituted into the formula (3a)below, into which the formula (2a) is solved for E^(β/α), to determine acorrection factor E^(β/α).E ^(β/α) ={A ^(β)×(1−F(β))}/(A ^(α) ×F(β))   (3a)

Measurements are made of respective IR spectrums of a plurality ofsamples having respective mixing ratios different from each other. Therespective correction factors E^(β/α) of the plurality of samples aredetermined by the above method, and the average of the correctionfactors E^(β/α) is then calculated.

(iii) Calculation of Respective Content Rates of Crystal Form α andCrystal Form β in Sample

For each sample, the content rate F(α) of the PVDF-based resin havingcrystal form α with respect to the crystal form α and crystal form βcombined is calculated on the basis of the average correction factorE^(β/α) calculated in (ii) above and the result of measurement of the IRspectrum of the sample.

Specifically, the content rate F(α) is calculated as follows: Alaminated body including the above porous layer is prepared by apreparation method described later. A portion of the laminated body iscut out for preparation of a measurement sample. Then, the infraredabsorption spectrum of the measurement sample at wave numbers from 4000cm⁻¹ to 400 cm⁻¹ (measurement range) is measured at room temperature(approximately 25° C.) with use of an FT-IR spectrometer (available fromBruker Optics K.K.; model: ALPHA Platinum-ATR) with a resolution of 4cm³¹ ¹ and 512 times of scanning. The measurement sample cut out ispreferably in the shape of an 80 mm×80 mm square. The size and shape ofthe measurement sample are, however, not limited to that; themeasurement sample simply needs to be so sized as to allow its infraredabsorption spectrum to be measured. Then, from the spectrum measured,the absorption intensity (A^(α)) at 765 cm⁻¹ (characteristic absorptionof crystal form α) and the absorption intensity (A^(β)) at 840 cm⁻¹(characteristic absorption of crystal form β) are determined. Thestarting point and end point of a waveform formed with the wave numberset as a peak are connected with a straight line, where the lengthbetween the straight line and the peak wave number (peak top) denote anabsorption intensity. For crystal form α, a maximum possible absorptionintensity within the wave number range of 775 cm⁻¹ to 745 cm⁻¹ isassumed to be the absorption intensity (A^(α)) at 765 cm⁻¹. For crystalform β, a maximum possible absorption intensity within the wave numberrange of 850 cm⁻¹ to 815 cm⁻¹ is assumed to be the absorption intensity(A^(β)) at 840 cm⁻¹. Note that the content rate F(α) of crystal form αherein is calculated on the assumption of the average correction factorE^(β/α) being 1.681 (with reference to Japanese Patent ApplicationPublication, Tokukai, No. 2005-200623). The calculation uses thefollowing formula (4a):F(α)(%)=[1-{absorption intensity (A ^(β)) at 840 cm⁻¹/(absorptionintensity (A ^(α)) at 765 cm⁻¹×correction factor (E ^(62 /α))(1.681)+absorption intensity (A ^(β)) at 840 cm⁻¹)}]×100   (4a)

<Method for Producing Porous Layer>

The porous layer for an embodiment of the present invention can beproduced by, for example, a method similar to a method for producing alaminated body and porous base material for an embodiment of the presentinvention.

<Method for Producing Porous Layer and Method for Producing LaminatedBody>

A porous layer and laminated body for an embodiment of the presentinvention may each be produced by any production method, and may each beproduced by any of various methods.

In an example method, a porous layer containing a PVDF-based resin andoptionally a filler is formed, through one of the processes (1) to (3)below, on a surface of a polyolefin-based resin microporous film to be aporous base material. In the case of the process (2) or (3), a porouslayer deposited is dried for removal of the solvent. In the processes(1) to (3), the coating solution, in the case of production of a porouslayer containing a filler, preferably contains a filler dispersedtherein and a PVDF-based resin dissolved therein.

The coating solution for use in a method for producing a porous layerfor an embodiment of the present invention can be prepared normally by(i) dissolving, in a solvent, a resin to be contained in the porouslayer for an embodiment of the present invention and (ii) dispersing, inthe solvent, fine particles to be contained in the porous layer for anembodiment of the present invention.

(1) A process of (i) coating a surface of a porous base material with acoating solution containing fine particles of a PVDF-based resin to becontained in a porous layer and optionally fine particles of a fillerand (ii) drying the surface of the porous base material to remove thesolvent (dispersion medium) from the coating solution for formation of aporous layer.

(2) A process of (i) coating a surface of a porous base material with acoating solution containing fine particles of a PVDF-based resin to becontained in a porous layer and optionally fine particles of a fillerand then (ii) immersing the porous base material into a depositionsolvent (which is a poor solvent for the PVDF-based resin) fordeposition of a porous layer containing the PVDF-based resin andoptionally the filler.

(3) A process of (i) coating a surface of a porous base material with acoating solution containing fine particles of a PVDF-based resin to becontained in a porous layer and optionally fine particles of a fillerand then (ii) making the coating solution acidic with use of alow-boiling-point organic acid for deposition of a porous layercontaining the PVDF-based resin and optionally the filler.

The solvent (dispersion medium) in the coating solution may be anysolvent that does not adversely affect the porous base material, thatallows a PVDF-based resin to be dissolved or dispersed therein uniformlyand stably, and that allows a filler to be dispersed therein uniformlyand stably. Examples of the solvent (dispersion medium) includeN-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide,acetone, and water.

The deposition solvent can be, for example, another solvent (hereinafteralso referred to as “solvent X”) that is dissolvable in the solvent(dispersion medium) contained in the coating solution and that does notdissolve the PVDF-based resin contained in the coating solution. Thesolvent (dispersion medium) can be efficiently removed from the coatingsolution by (i) immersing, in the solvent X, a porous base material towhich the coating solution has been applied and on which a coating filmhas been formed, for replacement of the solvent (dispersion medium) inthe coating film on the porous base material or a support with thesolvent X and then (ii) evaporating the solvent X. The depositionsolvent is preferably isopropyl alcohol or t-butyl alcohol, for example.

For the process (3), the low-boiling-point organic acid can be, forexample, paratoluene sulfonic acid or acetic acid.

The coating solution may be prepared by any method that allows thecoating solution to satisfy conditions such as the resin solid content(resin concentration) and the fine-particle amount that are necessary toproduce a desired porous layer. Specific examples of the method forpreparing a coating solution include a mechanical stirring method, anultrasonic dispersion method, a high-pressure dispersion method, and amedia dispersion method. The fine particles may be dispersed in thesolvent (dispersion medium) with use of a conventionally publicly knowndispersing device such as a three-one motor, a homogenizer, a media-typedispersing device, or a pressure-type dispersing device. Further, thecoating solution may be prepared simultaneously with wet grinding offine particles by supplying into a wet grinding device a liquid in whicha resin is dissolved or swollen or an emulsified liquid of a resinduring wet grinding carried out to produce fine particles having adesired average particle diameter. In other words, the wet grinding offine particles and the preparation of a coating solution may be carriedout simultaneously in a single step. The coating solution may contain anadditive(s) such as a dispersing agent, a plasticizing agent, a surfaceactive agent, and a pH adjusting agent as a component(s) other than theresin and the fine particles as long as such an additive does notprevent an object of the present invention from being attained. Theadditive may be added in an amount that does not prevent an object ofthe present invention from being attained.

The coating solution may be applied to the porous base material by anymethod, that is, a porous layer may be formed by any method on a surfaceof a porous base material that may have been subjected to ahydrophilization treatment as necessary. In a case where a porous layeris disposed on each of both surfaces of the porous base material, (i) asequential deposition method may be used, in which a porous layer isformed on one surface of the porous base material, and another porouslayer is subsequently formed on the other surface of the porous basematerial, or (ii) a simultaneous deposition method may be used, in whichporous layers are formed simultaneously on both surfaces of the porousbase material. A porous layer can be formed (that is, a laminated bodycan be produced) by, for example, (i) a method of applying the coatingsolution directly to a surface of the porous base material and thenremoving the solvent (dispersion medium), (ii) a method of applying thecoating solution to an appropriate support, removing the solvent(dispersion medium) for formation of a porous layer, thenpressure-bonding the porous layer to the porous base material, andpeeling the support off, (iii) a method of applying the coating solutionto a surface of an appropriate support, then pressure-bonding the porousbase material to that surface, then peeling the support off, and thenremoving the solvent (dispersion medium), or (iv) a method of immersingthe porous base material into the coating solution for dip coating andthen removing the solvent (dispersion medium). The thickness of theporous layer can be controlled by adjusting, for example, the thicknessof the coating film in a wet state (wet) after the coating, the weightratio between the resin and the fine particles, and the solid contentconcentration (that is, the sum of the resin concentration and thefine-particle concentration) of the coating solution. The support canbe, for example, a resin film, a metal belt, or a drum.

The coating solution may be applied to the porous base material orsupport by any method that can achieve a necessary weight per unit areaand a necessary coating area. The coating solution can be applied by aconventionally publicly known method. Specific examples include agravure coater method, a small-diameter gravure coater method, a reverseroll coater method, a transfer roll coater method, a kiss coater method,a dip coater method, a knife coater method, an air doctor blade coatermethod, a blade coater method, a rod coater method, a squeeze coatermethod, a cast coater method, a bar coater method, a die coater method,a screen printing method, and a spray coating method.

The solvent (dispersion medium) is typically removed by a drying method.Examples of the drying method include natural drying, air-blow drying,heat drying, and drying under reduced pressure. The solvent (dispersionmedium) can, however, be removed by any method that allows the solvent(dispersion medium) to be removed sufficiently. The solvent (dispersionmedium) contained in the coating solution may be replaced with anothersolvent before a drying operation. The solvent (dispersion medium) canbe replaced with another solvent for removal by, for example, a methodof (i) preparing another solvent (hereinafter referred to as “solventX”) that is dissolvable in the solvent (dispersion medium) contained inthe coating solution and that does not dissolve the resin contained inthe coating solution, (ii) immersing the porous base material orsupport, to which the coating solution has been applied and on which acoating film has been formed, into the solvent X to replace the solvent(disperse medium) in the coating film on the porous base material orsupport with the solvent X, and (iii) evaporating the solvent X. Thismethod allows the solvent (dispersion medium) to be removed efficientlyfrom the coating solution. In a case where the coating film, formed onthe porous base material or support by applying the coating solutionthereto, is heated when removing the solvent (dispersion medium) orsolvent X from the coating film, the coating film is desirably heated ata temperature that does not decrease the air permeability of the porousbase material, specifically within a range of 10° C. to 120° C.,preferably within a range of 20° C. to 80° C., to prevent pores in theporous base material from contracting to decrease the air permeabilityof the porous base material.

The solvent (dispersion medium) is preferably removed by, in particular,a method of applying the coating solution to a base material and thendrying the base material for formation of a porous layer. Thisarrangement makes it possible to produce a porous layer having a smallerporosity variation and fewer wrinkles.

The above drying can be carried out with the use of a normal dryingdevice.

The porous layer normally has, on one surface of the porous basematerial, an applied amount (weight per unit area) within a range ofpreferably 0.5 g/m² to 20 g/m², more preferably 0.5 g/m² to 10 g/m²,preferably 0.5 g/m² to 1.5 g/m², in terms of the solid content in viewof adhesiveness to an electrode and ion permeability. This means thatthe amount of the coating solution to be applied to the porous basematerial is preferably adjusted so that the porous layer in a laminatedbody or porous base material to be produced has an applied amount(weight per unit area) within the above range.

In a case where an additional layer such as a heat-resistant layer is tobe disposed on the laminated body, such a heat-resistant layer can bedisposed by a method similar to the above method except that the resinfor the porous layer is replaced with a resin for the heat-resistantlayer.

The present embodiment is arranged such that in any of the processes (1)to (3), changing the amount of resin for a porous layer which resin isto be dissolved or dispersed in a solution can adjust the volume ofresin that is contained per square meter of a porous layer havingundergone immersion in an electrolyte and that has absorbed theelectrolyte.

Further, changing the amount of solvent in which the resin for theporous layer is to be dissolved or dispersed can adjust the porosity andaverage pore diameter of a porous layer having undergone immersion in anelectrolyte.

<Method for Controlling Crystal Forms of PVDF-Based Resin>

A laminated body of an embodiment of the present invention is producedwhile adjustment is made of the drying conditions (for example, thedrying temperature, and the air velocity and direction during drying)and/or the deposition temperature (that is, the temperature at which aporous layer containing a PVDF-based resin is deposited with use of adeposition solvent or a low-boiling-point organic acid) for theabove-described method to control the crystal forms of the PVDF-basedresin to be contained in a porous layer to be formed. Specifically, alaminated body of an embodiment of the present invention can be producedwhile the drying conditions and the deposition temperature are adjustedso that the PVDF-based resin contains crystal form a in an amount of notless than 36 mol % (preferably not less than 39 mol %, more preferablynot less than 40 mol %, more preferably not less than 50 mol %, morepreferably not less than 60 mol %, further preferably not less than 70mol %; preferably not more than 95 mol %) with respect to 100 mol % ofthe total amount of the crystal form α and crystal form β contained.

The drying conditions and the deposition temperature, which are adjustedso that the PVDF-based resin contains crystal form α in an amount of notless than 36 mol % with respect to 100 mol % of the total amount of thecrystal form α and crystal form β contained, may be changed asappropriate in correspondence with, for example, the method forproducing a porous layer, the kind of solvent (dispersion medium) to beused, the kind of deposition solvent to be used, and/or the kind oflow-boiling-point organic acid to be used.

In a case where a deposition solvent is not used and the coatingsolution is simply dried as in the process (1), the drying conditionsmay be changed as appropriate in correspondence with, for example, theamount of the solvent in the coating solution, the concentration of thePVDF-based resin in the coating solution, the amount of the filler (ifcontained), and/or the amount of the coating solution to be applied. Ina case where a porous layer is to be formed through the process (1)described above, it is preferable that the drying temperature be 30° C.to 100° C., that the direction of hot air for drying be perpendicular toa porous base material or electrode sheet to which the coating solutionhas been applied, and that the velocity of the hot air be 0.1 m/s to 40m/s. Specifically, in a case where a coating solution to be appliedcontains N-methyl-2-pyrrolidone as the solvent for dissolving aPVDF-based resin, 1.0% by mass of a PVDF-based resin, and 9.0% by massof alumina as an inorganic filler, the drying conditions are preferablyadjusted so that the drying temperature is 40° C. to 100° C., that thedirection of hot air for drying is perpendicular to a porous basematerial or electrode sheet to which the coating solution has beenapplied, and that the velocity of the hot air is 0.4 m/s to 40 m/s.

In a case where a porous layer is to be formed through the process (2)described above, it is preferable that the deposition temperature be−25° C. to 60° C. and that the drying temperature be 20° C. to 100° C.Specifically, in a case where a porous layer is to be formed through theabove-described process (2) with use of N-methylpyrrolidone as thesolvent for dissolving a PVDF-based resin and isopropyl alcohol as thedeposition solvent, it is preferable that the deposition temperature be−10° C. to 40° C. and that the drying temperature be 30° C. to 80° C.

[2. Nonaqueous Electrolyte Secondary Battery Member and nonaqueousElectrolyte Secondary Battery]

A nonaqueous electrolyte secondary battery member in accordance with anembodiment of the present invention includes a cathode, a laminatedbody, and an anode that are arranged in this order. A nonaqueouselectrolyte secondary battery in accordance with an embodiment of thepresent invention includes a laminated body as a separator. Thedescription below deals with (i) a lithium-ion secondary battery memberas an example of a nonaqueous electrolyte secondary battery member and(ii) a lithium-ion secondary battery as an example of the nonaqueouselectrolyte secondary battery. The components of the nonaqueouselectrolyte secondary battery member and nonaqueous electrolytesecondary battery other than the above laminated body are not limited tothose described below.

A nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention can include a nonaqueous electrolytecontaining, for example, an organic solvent and a lithium salt dissolvedtherein. Examples of the lithium salt include LiClO₄, LiPF₆, LiAsF₆,LiSbF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, Li₂B₁₀Cl₁₀, loweraliphatic carboxylic acid lithium salt, and LiAlCl₄. The presentembodiment may use only one kind of the above lithium salts or two ormore kinds of the above lithium salts in combination. It is preferableto use, among the above lithium salts, at least one fluorine-containinglithium salt selected from the group consisting of LiPF₆, LiAsF₆,LiSbF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, and LiC(CF₃SO₂)₃.

Specific examples of the organic solvent in the nonaqueous electrolyteinclude carbonates such as ethylene carbonate, propylene carbonate,dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate,4-trifluoromethyl-1,3-dioxolane-2-one, and 1,2-di(methoxycarbonyloxy)ethane; ethers such as 1,2-dimethoxyethane,1,3-dimethoxypropane, pentafluoropropyl methylether,2,2,3,3-tetrafluoropropyl difluoro methylether, tetrahydrofuran, and2-methyl tetrahydrofuran; esters such as methyl formate, methyl acetate,and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile;amides such as N,N-dimethylformamide and N,N-dimethylacetamide;carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compoundssuch as sulfolane, dimethyl sulfoxide, and 1,3-propane sultone; andfluorine-containing organic solvents prepared by introducing a fluorinegroup into the organic solvents described above. The present embodimentmay use only one kind of the above organic solvents or two or more kindsof the above organic solvents in combination. Among the above organicsolvents, carbonates are preferable. A mixed solvent of a cycliccarbonate and an acyclic carbonate or a mixed solvent of a cycliccarbonate and an ether is further preferable. The mixed solvent of acyclic carbonate and an acyclic carbonate is further preferably a mixedsolvent of ethylene carbonate, dimethyl carbonate, and ethyl methylcarbonate because such a mixed solvent allows a wider operatingtemperature range, and is not easily decomposed even in a case where thepresent embodiment uses, as an anode active material, a graphitematerial such as natural graphite or artificial graphite.

The cathode is normally a sheet-shaped cathode including (i) a cathodemix containing a cathode active material, an electrically conductivematerial, and a binder and (ii) a cathode current collector supportingthe cathode mix thereon.

The cathode active material is, for example, a material capable of beingdoped and dedoped with lithium ions. Specific examples of such amaterial include a lithium complex oxide containing at least onetransition metal such as V, Mn, Fe, Co, or Ni. Among such lithiumcomplex oxides, (i) a lithium complex oxide having an α-NaFeO₂ structuresuch as lithium nickelate and lithium cobaltate and (ii) a lithiumcomplex oxide having a spinel structure such as lithium manganese spinelare preferable because such lithium complex oxides have a high averagedischarge potential. The lithium complex oxide may further contain anyof various metallic elements, and is further preferably complex lithiumnickelate. Further, the complex lithium nickelate particularlypreferably contains at least one metallic element selected from thegroup consisting of Ti, Zr, Ce, Y, V, Cr, Mn, Fe, Co, Cu, Ag, Mg, Al,Ga, In, and Sn at a proportion of 0.1 mol % to 20 mol % with respect tothe sum of the number of moles of the at least one metallic element andthe number of moles of Ni in the lithium nickelate. This is because sucha complex lithium nickelate allows an excellent cycle characteristic foruse in a high-capacity battery. Among others, an active material thatcontains Al or Mn and that contains Ni at a proportion of not less than85%, further preferably not less than 90%, is particularly preferablebecause a nonaqueous electrolyte secondary battery including a cathodecontaining the above active material has an excellent in cyclecharacteristic for use as a high-capacity battery.

Examples of the electrically conductive material include carbonaceousmaterials such as natural graphite, artificial graphite, cokes, carbonblack, pyrolytic carbons, carbon fiber, and a fired product of anorganic polymer compound. The present embodiment may use (i) only onekind of the above electrically conductive materials or (ii) two or morekinds of the above electrically conductive materials in combination, forexample a mixture of artificial graphite and carbon black.

Examples of the binder include thermoplastic resins such aspolyvinylidene fluoride, a copolymer of vinylidene fluoride,polytetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylenecopolymer, a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer,an ethylene-tetrafluoroethylene copolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, a thermoplastic polyimide,polyethylene, and polypropylene; an acrylic resin; and styrene-butadienerubber. The binder functions also as a thickening agent.

The cathode mix may be prepared by, for example, a method of applyingpressure to the cathode active material, the electrically conductivematerial, and the binder on the cathode current collector or a method ofusing an appropriate organic solvent so that the cathode activematerial, the electrically conductive material, and the binder are in apaste form.

Examples of the cathode current collector include electric conductorssuch as Al, Ni, and stainless steel. Among these, Al is preferable as itis easy to process into a thin film and less expensive.

The sheet-shaped cathode may be produced, that is, the cathode mix maybe supported by the cathode current collector, through, for example, amethod of applying pressure to the cathode active material, theelectrically conductive material, and the binder on the cathode currentcollector to form a cathode mix thereon or a method of (i) using anappropriate organic solvent so that the cathode active material, theelectrically conductive material, and the binder are in a paste form toprovide a cathode mix, (ii) applying the cathode mix to the cathodecurrent collector, (iii) drying the applied cathode mix to prepare asheet-shaped cathode mix, and (iv) applying pressure to the sheet-shapedcathode mix so that the sheet-shaped cathode mix is firmly fixed to thecathode current collector.

The anode is normally a sheet-shaped anode including (i) an anode mixcontaining an anode active material and (ii) an anode current collectorsupporting the anode mix thereon. The sheet-shaped anode preferablycontains the above electrically conductive material and binder.

The anode active material is, for example, (i) a material capable ofbeing doped and dedoped with lithium ions, (ii) a lithium metal, or(iii) a lithium alloy. Specific examples of the material includecarbonaceous materials such as natural graphite, artificial graphite,cokes, carbon black, pyrolytic carbons, carbon fiber, and a firedproduct of an organic polymer compound; chalcogen compounds such as anoxide and a sulfide that are doped and dedoped with lithium ions at anelectric potential lower than that for the cathode; metals such asaluminum (Al), lead (Pb), tin (Sn), bismuth (Bi), or silicon (Si), eachof which is alloyed with alkali metal; an intermetallic compound (AlSb,Mg₂Si, NiSi₂) of a cubic system in which intermetallic compound alkalimetal can be inserted in voids in a lattice; and a lithium nitrogencompound (Li₃−xM_(x)N (where M represents a transition metal)). Amongthe above anode active materials, a carbonaceous material containing agraphite material such as natural graphite or artificial graphite as amain component is preferable, an anode active material which is amixture of graphite and silicon and in which the ratio of Si to C is notless than 5% is more preferable, and an anode active material in whichthe ratio of Si to C is not less than 10% is further preferable. This isbecause such a carbonaceous material has high electric potentialflatness and low average discharge potential and can thus be combinedwith a cathode to achieve high energy density.

The anode mix may be prepared by, for example, a method of applyingpressure to the anode active material on the anode current collector ora method of using an appropriate organic solvent so that the anodeactive material is in a paste form.

The anode current collector is, for example, Cu, Ni, or stainless steel.Among these, Cu is preferable as it is not easily alloyed with lithiumin the case of a lithium-ion secondary battery in particular and iseasily processed into a thin film.

The sheet-shaped anode may be produced, that is, the anode mix may besupported by the anode current collector, through, for example, a methodof applying pressure to the anode active material on the anode currentcollector to form an anode mix thereon or a method of (i) using anappropriate organic solvent so that the anode active material is in apaste form to provide an anode mix, (ii) applying the anode mix to theanode current collector, (iii) drying the applied anode mix to prepare asheet-shaped anode mix, and (iv) applying pressure to the sheet-shapedanode mix so that the sheet-shaped anode mix is firmly fixed to theanode current collector. The above paste preferably includes aconductive aid and binder.

A nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention may be produced by (i) arranging thecathode, the laminated body, and the anode in this order so as to form anonaqueous electrolyte secondary battery member in accordance with anembodiment of the present invention, (ii) inserting the nonaqueouselectrolyte secondary battery member into a container that is for use asa housing of the nonaqueous electrolyte secondary battery, (iii) fillingthe container with a nonaqueous electrolyte, and (iv) hermeticallysealing the container under reduced pressure. The nonaqueous electrolytesecondary battery may have any shape such as the shape of a thin plate(sheet), a disk, a cylinder, or a prism such as a cuboid. The nonaqueouselectrolyte secondary battery may be produced by any method, and may beproduced by a conventionally publicly known method.

EXAMPLES

<Method for Measuring Various Physical Properties of Porous BaseMaterial>

Various physical properties of porous base materials in accordance withProduction Examples and Comparative Examples below were measured by thefollowing methods:

(1) Temperature Rise Ending Period at Microwave Irradiation

An 8 cm×8 cm test piece was cut out from a porous base material, and theweight W (g) of the test piece was measured. Then, the weight per unitarea of the test piece was calculated in accordance with the followingformula:Weight per unit area (g/m ²)=W/(0.08×0.08).

Next, the test piece was impregnated with N-methylpyrrolidone (NMP) towhich 3% by weight of water had been added. Then, the test piece wasplaced on a Teflon (registered trademark) sheet (12 cm×10 cm). The testpiece was folded in half in such a manner as to sandwich an opticalfiber thermometer (available from ASTEC Co., Ltd., Neoptix Reflexthermometer) coated with polytetrafluoroethylene (PTFE).

Next, the test piece, which had been impregnated with NMP containingwater and had been so folded as to sandwich the thermometer, was fixedin a microwave irradiation device (available from Micro Denshi Co.,Ltd., 9-kW microwave oven; frequency: 2455 MHz) equipped with aturntable. The test piece was then irradiated with a microwave at 1800 Wfor 2 minutes.

The optical fiber thermometer was used to measure, every 0.2 seconds,changes in the temperature of the test piece after the start of themicrowave irradiation. In the temperature measurements, the temperatureat which no temperature rise was measured for not less than 1 second wasused as a temperature rise ending temperature, and the time period thatelapsed before the temperature rise ending temperature was reached afterthe start of the microwave irradiation was used as a temperature riseending period. The temperature rise ending period was divided by theabove weight per unit area for calculation of a temperature rise endingperiod with respect to the amount of resin per unit area.

(2) Initial Rate Characteristic

Nonaqueous electrolyte secondary batteries each assembled as describedlater were each subjected to four cycles of initial charge anddischarge. Each of the four cycles of the initial charge and dischargewas carried out at 25° C., at a voltage ranging from 4.1 V to 2.7 V, andat an electric current value of 0.2 C. Note that the value of anelectric current at which a battery rated capacity defined as a one-hourrate discharge capacity was discharged in one hour was assumed to be 1C. This applies also to the following descriptions.

Each nonaqueous electrolyte secondary battery that had been subjected toinitial charge and discharge was subjected to three cycles of charge anddischarge at a constant charge electric current value of 1 C and aconstant discharge electric current value of 0.2 C at 55° C., and wasalso subjected to another three cycles of charge and discharge at aconstant charge electric current value of 1 C and a constant dischargeelectric current value of 20 C at 55° C. The ratio (20 C dischargecapacity/0.2 C discharge capacity) of (i) the discharge capacity in thethird cycle at the discharge electric current value of 20 C to (ii) thedischarge capacity in the third cycle at the discharge electric currentvalue of 0.2 C was calculated as the initial rate characteristic.

(3) Ratio of Maintenance of Rate Characteristic through Charge andDischarge Cycle

The nonaqueous electrolyte secondary batteries, which had been subjectedto the measurement of the initial rate characteristic, were eachsubjected to charge and discharge in which 100 cycles of charge anddischarge were carried out. Each of the 100 cycles of the charge anddischarge was carried out at 55° C., at a voltage ranging from 4.2 V to2.7 V, and at a constant charge electric current value of 1 C and aconstant discharge electric current value of 10 C.

Each nonaqueous electrolyte secondary battery, which had been subjectedto 100 cycles of charge and discharge, was subjected to three cycles ofcharge and discharge at a constant charge electric current value of 1 Cand a constant discharge electric current value of 0.2 C at 55° C., andwas also subjected to another three cycles of charge and discharge at aconstant charge electric current value of 1 C and a constant dischargeelectric current value of 20 C at 55° C. The ratio (20 C dischargecapacity/0.2 C discharge capacity) of (i) the discharge capacity in thethird cycle at the discharge electric current value of 20 C to (ii) thedischarge capacity in the third cycle at the discharge electric currentvalue of 0.2 C was calculated as a rate characteristic after 100 cyclesof charge and discharge (rate characteristic after 100 cycles).

On the basis of the results of the above rate characteristic tests, theratio (%) of maintenance of the rate characteristic through a charge anddischarge cycle in accordance with the following formula:Ratio of maintenance of rate characteristic=(rate characteristic after100 cycles)/(initial rate characteristic)×100

<Preparation of Porous Base Material>

Porous films in accordance with Production Examples 1 to 4 andComparative Examples 2 and 3 were prepared as described below for use asporous base materials.

Production Example 1

First, 68% by weight of ultra-high molecular weight polyethylene powder(GUR2024, available from Ticona Corporation; weight-average molecularweight: 4,970,000) and 32% by weight of polyethylene wax (FNP-0115;available from Nippon Seiro Co., Ltd.) having a weight-average molecularweight of 1000 were prepared, that is, 100 parts by weight in total ofthe ultra-high molecular weight polyethylene and the polyethylene waxwere prepared. Then, 0.4% by weight of an antioxidant (Irg1010,available from Ciba Specialty Chemicals), 0.1% by weight of anantioxidant (P168, available from Ciba Specialty Chemicals), and 1.3% byweight of sodium stearate were added to the ultra-high molecular weightpolyethylene and the polyethylene wax, and then calcium carbonate(available from Maruo Calcium Co., Ltd.) having an average pore diameterof 0.1 μm was further added by 38% by volume with respect to the totalvolume of the above ingredients. Then, the ingredients were mixed inpowder form with use of a Henschel mixer, and were then melted andkneaded with use of a twin screw kneading extruder. This produced apolyolefin resin composition. Then, the polyolefin resin composition wasrolled with use of a pair of rollers each having a surface temperatureof 150° C. into a sheet. This sheet was immersed in an aqueoushydrochloric acid solution (containing 4 mol/L of hydrochloric acid and0.5% by weight of nonionic surfactant) for removal of the calciumcarbonate, and was then stretched 6.2-fold at 100° C. to 105° C. at astraining rate of 1250% per minute. This prepared a film having athickness of 10.9 μm. This film was then subjected to a heat-fixationtreatment at 126° C. This produced a porous base material of ProductionExample 1.

Production Example 2

First, 70% by weight of ultra-high molecular weight polyethylene powder(GUR4032, available from Ticona Corporation; weight-average molecularweight: 4,970,000) and 30% by weight of polyethylene wax (FNP-0115;available from Nippon Seiro Co., Ltd.) having a weight-average molecularweight of 1000 were prepared, that is, 100 parts by weight in total ofthe ultra-high molecular weight polyethylene and the polyethylene waxwere prepared. Then, 0.4% by weight of an antioxidant (Irg1010,available from Ciba Specialty Chemicals), 0.1% by weight of anantioxidant (P168, available from Ciba Specialty Chemicals), and 1.3% byweight of sodium stearate were added to the ultra-high molecular weightpolyethylene and the polyethylene wax, and then calcium carbonate(available from Maruo Calcium Co., Ltd.) having an average pore diameterof 0.1 μm was further added by 36% by volume with respect to the totalvolume of the above ingredients. Then, the ingredients were mixed inpowder form with use of a Henschel mixer, and were then melted andkneaded with use of a twin screw kneading extruder. This produced apolyolefin resin composition. Then, the polyolefin resin composition wasrolled with use of a pair of rollers each having a surface temperatureof 150° C. into a sheet. This sheet was immersed in an aqueoushydrochloric acid solution (containing 4 mol/L of hydrochloric acid and0.5% by weight of nonionic surfactant) for removal of the calciumcarbonate, and was then stretched 6.2-fold at 100° C. to 105° C. at astraining rate of 1250% per minute. This prepared a film having athickness of 15.5 μm. This film was then subjected to a heat-fixationtreatment at 120° C. This produced a porous base material of ProductionExample 2.

Production Example 3

First, 71% by weight of ultra-high molecular weight polyethylene powder(GUR4032, available from Ticona Corporation; weight-average molecularweight: 4,970,000) and 29% by weight of polyethylene wax (FNP-0115;available from Nippon Seiro Co., Ltd.) having a weight-average molecularweight of 1000 were prepared, that is, 100 parts by weight in total ofthe ultra-high molecular weight polyethylene and the polyethylene waxwere prepared. Then, 0.4% by weight of an antioxidant (Irg1010,available from Ciba Specialty Chemicals), 0.1% by weight of anantioxidant (P168, available from Ciba Specialty Chemicals), and 1.3% byweight of sodium stearate were added to the ultra-high molecular weightpolyethylene and the polyethylene wax, and then calcium carbonate(available from Maruo Calcium Co., Ltd.) having an average pore diameterof 0.1 μm was further added by 37% by volume with respect to the totalvolume of the above ingredients. Then, the ingredients were mixed inpowder form with use of a Henschel mixer, and were then melted andkneaded with use of a twin screw kneading extruder. This produced apolyolefin resin composition. Then, the polyolefin resin composition wasrolled with use of a pair of rollers each having a surface temperatureof 150° C. into a sheet. This sheet was immersed in an aqueoushydrochloric acid solution (containing 4 mol/L of hydrochloric acid and0.5% by weight of nonionic surfactant) for removal of the calciumcarbonate, and was then stretched 7.0-fold at 100° C. to 105° C. at astraining rate of 2100% per minute. This prepared a film having athickness of 11.7 μm. This film was then subjected to a heat-fixationtreatment at 123° C. This produced a porous base material of ProductionExample 3.

Production Example 4

First, 70% by weight of ultra-high molecular weight polyethylene powder(GUR4032, available from Ticona Corporation; weight-average molecularweight: 4,970,000) and 30% by weight of polyethylene wax (FNP-0115;available from Nippon Seiro Co., Ltd.) having a weight-average molecularweight of 1000 were prepared, that is, 100 parts by weight in total ofthe ultra-high molecular weight polyethylene and the polyethylene waxwere prepared. Then, 0.4% by weight of an antioxidant (Irg1010,available from Ciba Specialty Chemicals), 0.1% by weight of anantioxidant (P168, available from Ciba Specialty Chemicals), and 1.3% byweight of sodium stearate were added to the ultra-high molecular weightpolyethylene and the polyethylene wax, and then calcium carbonate(available from Maruo Calcium Co., Ltd.) having an average pore diameterof 0.1 μm was further added by 36% by volume with respect to the totalvolume of the above ingredients. Then, the ingredients were mixed inpowder form with use of a Henschel mixer, and were then melted andkneaded with use of a twin screw kneading extruder. This produced apolyolefin resin composition. Then, the polyolefin resin composition wasrolled with use of a pair of rollers each having a surface temperatureof 150° C. into a sheet. This sheet was immersed in an aqueoushydrochloric acid solution (containing 4 mol/L of hydrochloric acid and0.5% by weight of nonionic surfactant) for removal of the calciumcarbonate, and was then stretched 6.2-fold at 100° C. to 105° C. at astraining rate of 750% per minute. This prepared a film having athickness of 16.3 μm. This film was then subjected to heat fixation at115° C. This produced a porous base material of Production Example 4.

Comparative Example 1

A commercially available polyolefin porous film (olefin separator) wasused as a porous base material of Comparative Example 1.

Comparative Example 2

First, 70% by weight of ultra-high molecular weight polyethylene powder(GUR4032, available from Ticona Corporation; weight-average molecularweight: 4,970,000) and 30% by weight of polyethylene wax (FNP-0115;available from Nippon Seiro Co., Ltd.) having a weight-average molecularweight of 1000 were prepared, that is, 100 parts by weight in total ofthe ultra-high molecular weight polyethylene and the polyethylene waxwere prepared. Then, 0.4% by weight of an antioxidant (Irg1010,available from Ciba Specialty Chemicals), 0.1% by weight of anantioxidant (P168, available from Ciba Specialty Chemicals), and 1.3% byweight of sodium stearate were added to the ultra-high molecular weightpolyethylene and the polyethylene wax, and then calcium carbonate(available from Maruo Calcium Co., Ltd.) having an average pore diameterof 0.1 μm was further added by 36% by volume with respect to the totalvolume of the above ingredients. Then, the ingredients were mixed inpowder form with use of a Henschel mixer, and were then melted andkneaded with use of a twin screw kneading extruder. This produced apolyolefin resin composition. Then, the polyolefin resin composition wasrolled with use of a pair of rollers each having a surface temperatureof 150° C. into a sheet. This sheet was immersed in an aqueoushydrochloric acid solution (containing 4 mol/L of hydrochloric acid and0.5% by weight of nonionic surfactant) for removal of the calciumcarbonate, and was then stretched 6.2-fold at 100° C. to 105° C. at astraining rate of 2000% per minute. This prepared a film having athickness of 16.3 μm. This film was then subjected to heat fixation at123° C. This produced a porous base material of Comparative Example 2.

Comparative Example 3

First, 71% by weight of ultra-high molecular weight polyethylene powder(GUR4032, available from Ticona Corporation; weight-average molecularweight: 4,970,000) and 29% by weight of polyethylene wax (FNP-0115;available from Nippon Seiro Co., Ltd.) having a weight-average molecularweight of 1000 were prepared, that is, 100 parts by weight in total ofthe ultra-high molecular weight polyethylene and the polyethylene waxwere prepared. Then, 0.4% by weight of an antioxidant (Irg1010,available from Ciba Specialty Chemicals), 0.1% by weight of anantioxidant (P168, available from Ciba Specialty Chemicals), and 1.3% byweight of sodium stearate were added to the ultra-high molecular weightpolyethylene and the polyethylene wax, and then calcium carbonate(available from Maruo Calcium Co., Ltd.) having an average pore diameterof 0.1 μm was further added by 37% by volume with respect to the totalvolume of the above ingredients. Then, the ingredients were mixed inpowder form with use of a Henschel mixer, and were then melted andkneaded with use of a twin screw kneading extruder. This produced apolyolefin resin composition. Then, the polyolefin resin composition wasrolled with use of a pair of rollers each having a surface temperatureof 150° C. into a sheet. This sheet was immersed in an aqueoushydrochloric acid solution (containing 4 mol/L of hydrochloric acid and0.5% by weight of nonionic surfactant) for removal of the calciumcarbonate, and was then stretched 7.1-fold at 100° C. to 105° C. at astraining rate of 750% per minute. This prepared a film having athickness of 11.5 μm. This film was then subjected to heat fixation at128° C. This produced a porous base material of Comparative Example 3.

Table 1 below shows the stretch straining rate, the film thickness afterstretching, the heat-fixation temperature, and the heat-fixationtemperature/film thickness after stretching (that is, heat-fixationtemperature per unit thickness of the stretched film) of each ofProduction Examples 1 to 4 and Comparative Examples 2 and 3.

TABLE 1 Heat-fixation Stretch Film thickness temperature/film strainingafter Heat-fixation thickness after rate stretching temperaturestretching [%/min] [μm] [° C.] [° C./μm] PE 1 1250 10.9 126 11.6 PE 21250 15.5 120 7.7 PE 3 2100 11.7 123 10.5 PE 4 750 16.3 115 7.1 CE 22000 16.3 123 7.5 CE 3 750 11.5 128 11.1 “PE” stands for ProductionExample “CE” stands for Comparative Example

<Preparation of Nonaqueous Electrolyte Secondary Battery>

Next, nonaqueous electrolyte secondary batteries were prepared as belowthat included the respective porous base materials prepared as above inProduction Examples 1 to 4 and Comparative Examples 1 to 3.

(Cathode)

A commercially available cathode was used that was produced by applyingLiNi_(0.5)Mn_(0.3)Co_(0.2)O₂/electrically conductive material/PVDF(weight ratio 92:5:3) to an aluminum foil. The aluminum foil waspartially cut off so that a cathode active material layer was present inan area of 45 mm×30 mm and that that area was surrounded by an area witha width of 13 mm in which area no cathode active material layer waspresent. The cutoff was used as a cathode. The cathode active materiallayer had a thickness of 58 μm and a density of 2.50 g/cm³. The cathodehad a capacity of 174 mAh/g.

(Anode)

A commercially available anode was used that was produced by applyinggraphite/styrene-1,3-butadiene copolymer/sodium carboxymethylcellulose(weight ratio 98:1:1) to a copper foil. The copper foil was partiallycut off so that an anode active material layer was present in an area of50 mm×35 mm and that that area was surrounded by an area with a width of13 mm in which area no anode active material layer was present. Thecutoff was used as an anode. The anode active material layer had athickness of 49 μm and a density of 1.40 g/cm³. The anode had a capacityof 372 mAh/g.

(Assembly)

In a laminate pouch, the cathode, the porous base material, and theanode were laminated (arranged) in this order so as to obtain anonaqueous electrolyte secondary battery member. During this operation,the cathode and the anode were arranged so that the cathode activematerial layer of the cathode had a main surface that was entirelycovered by the main surface of the anode active material layer of theanode.

Subsequently, the nonaqueous electrolyte secondary battery member wasput into a bag made of a laminate of an aluminum layer and a heat seallayer. Further, 0.25 mL of nonaqueous electrolyte was put into the bag.The nonaqueous electrolyte was an electrolyte at 25° C. prepared bydissolving LiPF₆ with a concentration of 1.0 mole per liter in a mixedsolvent of ethyl methyl carbonate, diethyl carbonate, and ethylenecarbonate in a volume ratio of 50:20:30. The bag was then heat-sealedwhile the pressure inside the bag was reduced. This produced anonaqueous electrolyte secondary battery. The nonaqueous electrolytesecondary battery had a design capacity of 20.5 mAh.

<Results of Measurement of Various Physical Properties>

Table 2 shows the results of measuring various physical properties ofeach of the porous base materials of Production Examples 1 to 4 andComparative Examples 1 to 3.

TABLE 2 Temperature Weight rise ending Ratio of per Temperatureperiod/weight Rate maintenance unit area rise ending per unit areaInitial rate characteristic of rate (g/cm²) period (sec) (sec · m²/g)characteristic after 100 cycles characteristic (%) PE 1 6.4 19.8 3.090.597 0.374 63 PE 2 6.9 20.6 2.99 0.771 0.523 68 PE 3 5.4 28.4 5.260.784 0.555 71 PE 4 5.3 29.8 5.62 0.840 0.493 59 CE 1 13.9 26.6 1.910.482 0.177 37 CE 2 9.6 25.8 2.69 0.141 0.127 90 CE 3 6.2 17.8 2.880.691 0.358 52 “PE” stands for Production Example “CE” stands forComparative Example

Table 2 shows that the respective porous base materials of ProductionExamples 1 to 4, in each of which the temperature rise ending periodwith respect to the amount of resin per unit area (weight per unit area)was 2.9 seconds·m²/g to 5.7 seconds·m²/g, each had an excellent initialrate characteristic and a smaller decrease in the rate of maintenance ofthe rate characteristic. This indicates that the respective porous basematerials of Production Examples 1 to 4 were superior to those ofComparative Examples 1 to 3, in each of which the temperature riseending period with respect to the weight per unit area was outside therange of 2.9 seconds·m²/g to 5.7 seconds·m²/g.

[Various Methods for Measuring Physical Properties of Laminated Body]

In the Examples and Comparative Examples below, properties such as the αrate and curl property were measured by the following methods:

(1) Method for Calculating α Rate

An α rate (%) was measured by the method below, the α rate (%) being amolar ratio (%) of crystal form α in the PVDF-based resin contained inthe porous layer in the laminated body produced in each of the Examplesand Comparative Examples below with respect to the total amount of thecrystal form α and crystal form β contained in the PVDF-based resin.

An 80 mm×80 mm square was cut out from the laminated body. The infraredabsorption spectrum of the cutout at wave numbers from 4000 cm⁻¹ to 400cm⁻¹ (measurement range) was measured at room temperature (approximately25° C.) with use of an FT-IR spectrometer (available from Bruker OpticsK.K.; model: ALPHA Platinum-ATR) with a resolution of 4 cm⁻¹ and 512times of scanning. Then, from the spectrum measured, the absorptionintensity at 765 cm⁻¹ (characteristic absorption of crystal form α) andthe absorption intensity at 840 cm⁻¹ (characteristic absorption ofcrystal form β) were determined. The starting point and end point of awaveform formed with the wave number set as a peak were connected with astraight line, where the length between the straight line and the peakwave number (peak top) denoted an absorption intensity. For crystal formα, a maximum possible absorption intensity within the wave number rangeof 775 cm⁻¹ to 745 cm⁻¹ was assumed to be the absorption intensity at765 cm⁻¹. For crystal form β, a maximum possible absorption intensitywithin the wave number range of 850 cm⁻¹ to 815 cm⁻¹ was assumed to bethe absorption intensity at 840 cm⁻¹.

The α rate was calculated as described above in accordance with theFormula (4a) below on the basis of a value obtained by (i) determiningthe absorption intensity at 765 cm⁻¹ corresponding to crystal form α andthe absorption intensity at 840 cm⁻¹ corresponding to crystal form β and(ii) multiplying the absorption intensity of crystal form α by 1.681(correction factor) with reference to Japanese Patent ApplicationPublication, Tokukai, No. 2005-200623.α rate (%)=[1−{absorption intensity at 840 cm⁻¹/(absorption intensity at765 cm⁻¹×correction factor (1.681)+absorption intensity at 840 cm⁻¹)}]×100   (4a)

(2) Curl Measurement

An 8 cm×8 cm square was cut out from the laminated body. The cutout waskept at room temperature (approximately 25° C.) and at a dew point of−30° C. for one (1) day. The appearance of the cutout was then evaluatedon the basis of the following criterion: The rate “C” represents a stateof a complete curl, the rates “A” and “B” each represent a better state,and the rate “A” represents the most preferable state.

A: No curved ends

B: Although an end(s) is curved, the remaining portion is mostly notcurved and is flat.

C: Opposite ends curved into a tube shape

Example 1

An N-methyl-2-pyrrolidone (hereinafter referred to also as “NMP”)solution (available from Kureha Corporation; product name: L#9305,weight-average molecular weight: 1,000,000) containing a PVDF-basedresin (polyvinylidene fluoride-hexafluoropropylene copolymer) wasprepared as a coating solution. The coating solution was applied by adoctor blade method to the porous base material produced in ProductionExample 1 so that the applied coating solution weighed 6.0 g per squaremeter of the PVDF-based resin in the coating solution. The porous film,to which the coating solution had been applied, was immersed into2-propanol while the coating film was wet with the solvent, and was thenleft to stand still at 25° C. for 5 minutes. This produced a laminatedporous film (1-i). The laminated porous film (1-i) produced was furtherimmersed into other 2-propanol while the laminated porous film (1-i) waswet with the above immersion solvent, and was then left to stand stillat 25° C. for 5 minutes. This produced a laminated porous film (1-ii).The laminated porous film (1-ii) produced was dried at 65° C. for 5minutes. This produced a laminated body (1). Table 3 shows the resultsof evaluation of the laminated body (1) produced.

Example 2

A laminated body (2) was prepared by a method similar to the method usedin Example 1 except that the porous base material prepared in ProductionExample 2 was used. Table 3 shows the results of evaluation of thelaminated body (2) produced.

Example 3

A laminated body (3) was prepared by a method similar to the method usedin Example 1 except that the porous base material prepared in ProductionExample 3 was used. Table 3 shows the results of evaluation of thelaminated body (3) produced.

Example 4

A laminated body (4) was prepared by a method similar to the method usedin Example 1 except that the porous base material prepared in ProductionExample 4 was used. Table 3 shows the results of evaluation of thelaminated body (4) produced.

Example 5

A porous film to which a coating solution had been applied as in Example1 was immersed into 2-propanol while the coating film was wet with thesolvent, and was then left to stand still at 0° C. for 5 minutes. Thisproduced a laminated porous film (5-i). The laminated porous film (5-i)produced was further immersed into other 2-propanol while the laminatedporous film (5-i) was wet with the above immersion solvent, and was thenleft to stand still at 25° C. for 5 minutes. This produced a laminatedporous film (5-ii). The laminated porous film (5-ii) thus obtained wasdried at 30° C. for 5 minutes, so that a laminated body (5) wasobtained. Table 3 shows the results of evaluation of the laminated body(5) produced.

Example 6

A porous film to which a coating solution had been applied as in Example2 was treated by a method similar to the method used in Example 5. Thisproduced a laminated body (6). Table 3 shows the results of evaluationof the laminated body (6) produced.

Example 7

A porous film to which a coating solution had been applied as in Example3 was treated by a method similar to the method used in Example 5. Thisproduced a laminated body (7). Table 3 shows the results of evaluationof the laminated body (7) produced.

Example 8

A porous film to which a coating solution had been applied as in Example4 was treated by a method similar to the method used in Example 5. Thisproduced a laminated body (8). Table 3 shows the results of evaluationof the laminated body (8) produced.

Example 9

A porous film to which a coating solution had been applied as in Example1 was immersed into 2-propanol while the coating film was wet with thesolvent, and was then left to stand still at −5° C. for 5 minutes. Thisproduced a laminated porous film (9-i). The laminated porous film (9-i)produced was further immersed into other 2-propanol while the laminatedporous film (9-i) was wet with the above immersion solvent, and was thenleft to stand still at 25° C. for 5 minutes. This produced a laminatedporous film (9-ii). The laminated porous film (9-ii) thus obtained wasdried at 30° C. for 5 minutes, so that a laminated body (9) wasobtained. Table 3 shows the results of evaluation of the laminated body(9) produced.

Example 10

A porous film to which a coating solution had been applied as in Example2 was treated by a method similar to the method used in Example 9. Thisproduced a laminated body (10). Table 3 shows the results of evaluationof the laminated body (10) produced.

Example 11

A porous film to which a coating solution had been applied as in Example3 was treated by a method similar to the method used in Example 9. Thisproduced a laminated body (11). Table 3 shows the results of evaluationof the laminated body (11) produced.

Example 12

A porous film to which a coating solution had been applied as in Example4 was treated by a method similar to the method used in Example 9. Thisproduced a laminated body (12). Table 3 shows the results of evaluationof the laminated body (12) produced.

Example 13

A porous film to which a coating solution had been applied as in Example2 was immersed into 2-propanol while the coating film was wet with thesolvent, and was then left to stand still at −10° C. for 5 minutes. Thisproduced a laminated porous film (13-i). The laminated porous film(13-i) produced was further immersed into other 2-propanol while thelaminated porous film (13-i) was wet with the above immersion solvent,and was then left to stand still at 25° C. for 5 minutes. This produceda laminated porous film (13-ii). The laminated porous film (13-ii)produced was dried at 30° C. for 5 minutes. This produced a laminatedbody (13). Table 3 shows the results of evaluation of the laminated body(13) produced.

Example 14

A porous film to which a coating solution had been applied as in Example3 was treated by a method similar to the method used in Example 13. Thisproduced a laminated body (14). Table 3 shows the results of evaluationof the laminated body (15) produced.

Example 15

A porous film to which a coating solution had been applied as in Example4 was treated by a method similar to the method used in Example 13. Thisproduced a laminated body (15). Table 3 shows the results of evaluationof the laminated body (15) produced.

Comparative Example 4

A porous film to which a coating solution had been applied as in Example1 was immersed into 2-propanol while the coating film was wet with thesolvent, and was then left to stand still at −78° C. for 5 minutes. Thisproduced a laminated porous film (17-i). The laminated porous film(17-i) produced was further immersed into other 2-propanol while thelaminated porous film (17-i) was wet with the above immersion solvent,and was then left to stand still at 25° C. for 5 minutes. This produceda laminated porous film (17-ii). The laminated porous film (17-ii) thusobtained was dried at 30° C. for 5 minutes, so that a laminated body(16) was obtained. Table 3 shows the results of evaluation of thelaminated body (16) produced.

Comparative Example 5

A porous film to which a coating solution had been applied as in Example2 was treated by a method similar to the method used in ComparativeExample 4. This produced a laminated body (17). Table 3 shows theresults of evaluation of the laminated body (17) produced.

Comparative Example 6

A porous film to which a coating solution had been applied as in Example3 was treated by a method similar to the method used in ComparativeExample 4. This produced a laminated body (18). Table 3 shows theresults of evaluation of the laminated body (18) produced.

Comparative Example 7

A porous film to which a coating solution had been applied as in Example4 was treated by a method similar to the method used in ComparativeExample 4. This produced a laminated body (19). Table 3 shows theresults of evaluation of the laminated body (19) produced.

TABLE 3 α rate (%) Curl measurement Example 1 100 A Example 2 100 AExample 3 100 A Example 4 94 A Example 5 84 A Example 6 87 A Example 792 A Example 8 80 A Example 9 63 A Example 10 74 A Example 11 78 AExample 12 64 A Example 13 36 B Example 14 45 A Example 15 39 AComparative Example 4 21 C Comparative Example 5 29 C ComparativeExample 6 27 C Comparative Example 7 25 C

[Results]

For the laminated bodies (1) to (15), which were produced in Examples 1to 15 and each of which included a porous layer containing a PVDF-basedresin that contained crystal form α in an amount (α rate) of not lessthan 36% with respect to the crystal form α and crystal form β combined,the measurement results show that curls were prevented. On the otherhand, for the laminated bodies (16) to (19), which were produced inComparative Examples 4 to 7 and for each of which the α rate was lessthan 36%, the measurement results show that clear curls occurred.

The above indicates that a laminated body in accordance with anembodiment of the present invention which laminated body has an α rateof not less than 36% is not easily curled.

There is a relationship between (i) the structure of pores of a porousbase material (namely, the capillary force in the pores and the area ofthe wall of the pores) in a laminated body and the capability to supplyan electrolyte from the porous base material to the electrodes and (ii)how the rate characteristic is degraded as the battery is charged anddischarged repeatedly and/or the battery is used with a large electriccurrent. In other words, the rate characteristic of the laminated bodydepends on the properties of the porous base material. The laminatedbodies produced in Examples 1 to 15 were each produced with use of theporous base material produced in one of Production Examples 1 to 4. Asshown in Table 2, a nonaqueous electrolyte secondary battery includingthe porous base material produced in any of Production Examples 1 to 4showed an excellent rate characteristic. It follows that a nonaqueouselectrolyte secondary battery including the laminated body produced inany of Examples 1 to 15 understandably each showed an excellent ratecharacteristic as well.

The results of Production Examples, Examples, and Comparative Examplesdescribed above show that the laminated bodies produced in Examples 1 to15 (laminated bodies in accordance with an embodiment of the presentinvention) can each impart an excellent rate characteristic to anonaqueous electrolyte secondary battery including the laminated body asa separator and are not easily curled.

INDUSTRIAL APPLICABILITY

A laminated body in accordance with an embodiment of the presentinvention is not easily curled, and is suitably usable in a nonaqueouselectrolyte secondary battery.

The invention claimed is:
 1. A laminated body, comprising: a porous basematerial containing a polyolefin-based resin as a main component; and aporous layer on at least one surface of the porous base material, theporous layer containing a polyvinylidene fluoride-based resin, theporous base material having a temperature rise ending period of 2.9seconds·m²/g to 5.7 seconds·m²/g with respect to an amount of resin perunit area in a case where the porous base material has been impregnatedwith N-methylpyrrolidone containing 3% by weight of water and has thenbeen irradiated with a microwave having a frequency of 2455 Hz and anoutput of 1800 W, the polyvinylidene fluoride-based resin containingcrystal form α in an amount of not less than 36 mol % with respect to100 mol % of a total amount of the crystal form α and crystal form βcontained in the polyvinylidene fluoride-based resin, where the amountof the crystal form α is calculated from an absorption intensity ataround 765 cm⁻¹ in an IR spectrum of the porous layer, and an amount ofthe crystal form β is calculated from an absorption intensity at around840 cm⁻¹ in the IR spectrum of the porous layer.
 2. The laminated bodyaccording to claim 1, wherein the polyvinylidene fluoride-based resincontains (i) a homopolymer of vinylidene fluoride and/or (ii) acopolymer of vinylidene fluoride and at least one monomer selected fromthe group consisting of hexafluoropropylene, tetrafluoroethylene,trifluoroethylene, trichloroethylene, and vinyl fluoride.
 3. Thelaminated body according to claim 1, wherein the polyvinylidenefluoride-based resin has a weight-average molecular weight of not lessthan 200,000 and not more than 3,000,000.
 4. The laminated bodyaccording to claim 1, wherein the porous layer contains a filler.
 5. Thelaminated body according to claim 4, wherein the filler has avolume-average particle size of not less than 0.01 μm and not more than10 μm.
 6. A nonaqueous electrolyte secondary battery member, comprising:a cathode; a laminated body according to claim 1; and an anode, thecathode, the laminated body, and the anode being arranged in this order.7. A nonaqueous electrolyte secondary battery, comprising as a separatora laminated body according to claim 1.